Liquid paint composition is classified into the two types: organic solvent type paint composition and aqueous paint composition. Aqueous paint composition is generally explained to be lower environmental load in comparison with organic solvent type paint composition, because it contains organic solvent in a small amount. However, even in case of aqueous paint composition, organic solvent is employed in many cases in order to control viscosity when preparing film forming resin or curing agent resin or when preparing paint, or in order to make work of preparing paint easy. If the organic solvent employed when preparing aqueous paint composition is also emitted into environment, the organic solvent also burdens on environment and therefore is not preferred.
For example, in case of preparing a cationic electrodeposition paint composition which is one of aqueous paint composition, it has been essential that organic solvent for diluting is formulated thereto when preparing cationic epoxy resin emulsion. Organic solvent is added for reducing viscosity of resin component and for improving handleability both in mixing when preparing an emulsion and in transferring a varnish. As the organic solvent for diluting, methyl isobutyl ketone (hereinafter called “MIBK”) and/or xylene are preferably used. This is because MIBK and xylene have compatibility with cationic epoxy resin and do not react with an epoxy group when preparing cationic epoxy resin. The presence of a large amount of MIBK and xylene for diluting in the electrodeposition paint composition severely affects paint stability and throwing power in electrocoating. Accordingly, the organic solvent is generally removed in a desolvation step which is one step of preparing emulsion resin. The removal of the solvent takes a time when preparing emulsion resin and the use of the solvent which is not necessary in a final paint increases preparation cost of electrodeposition paint composition.
For example, JP 2004-307774 A (Patent Literature 1) discloses a process for producing a cationic electrodeposition paint from a cationic paint resin composition which comprises a basic resin, a curing agent, an additive and an organic solvent, wherein (1) the additive is formulated in water dispersion and desolvation and (2) after a neutralization treatment, deionized water is added and pressure is reduced, whereby both of the water dispersion step and the desolvation step are carried out simultaneously (see claim 1). The process of Patent Literature 1, however, is not preferred in view of reduction of VOC (volatile organic compound), because a large amount of organic solvent is employed in the process of Patent Literature 1. Desolvation step is also necessary to remove the organic solvent and the process of Patent Literature 1 does not lead to reduction of production time of paint and to elimination of production equipment.
Another approach to reduce organic solvent and to enhance handleability by reducing viscosity of resin components includes, for example, a means in which mixing operation is conducted at an elevated temperature of 100° C. or more. It is general that a resin component reduces its viscosity, as temperature increases. However, an epoxy resin and a curing agent of the resin components generally have reactivity with each other over a certain temperature and start reaction. As the result of the reaction, a molecular weight of the resin components increases and a viscosity of the composition increases adversely, which deteriorates smoothness of the resulting coating film. Accordingly, the approach to increase temperature and to decrease viscosity does not apply directly a preparation of a heat-curable paint.
JP 2009-138126 A (Patent Literature 2) discloses a method for preparing a cationic electrodeposition paint composition which comprises: the first step of emulsifying an amine-modified epoxy resin (a), a thermosetting agent (b) and an acrylic monomer (c) to form a pre-emulsion, the second step of adding a polymerization initiator to the pre-emulsion and polymerizing the acrylic monomer (c) to obtain an acrylic resin-containing emulsion, and the third step of mixing the emulsion obtained at the second step with a pigment dispersion paste to prepare the cationic electrodeposition coating composition (see claim 1 and other portions of Patent Literature 2). Patent Literature 2 discloses in paragraphs [0024] to [0026] that the method can surely make possible leveling function derived from the acrylic resin component in the cationic electrodeposited coating and can effectively cut an amount of volatile organic compound (VOC) in preparing the cationic electrodeposition paint composition. In the invention of Patent Literature 2, the acrylic monomer is emulsified to form a pre-emulsion, but the present invention does not form pre-emulsion.